1. Field of the Invention
The present invention relates to a spindle motor used in a magnetic recording apparatus such as a hard disk drive (HDD), and more particularly to a spindle motor having a bearing which requires high rotational accuracy.
2. Description of the Related Art
Conventionally, in conjunction with trends toward a higher storage capacity and a shorter access time of a rotatingly driven portion, e.g., a hard disk drive (HDD), of a magnetic storage device, a spindle motor used for driving the HDD is correspondingly required to rotate at high speed and with high accuracy. Namely, a spindle motor having a bearing device adapted to such high-speed high-accuracy rotation is required.
As bearings adapted to such high-speed high-accuracy rotation, hydrodynamic fluid radial bearings are known, and a hydrodynamic oil radial bearing, in particular, is used as a bearing having a large load carrying capacity and suited to miniaturization. As a spindle motor in which a hydrodynamic oil radial bearing is adopted, one disclosed in, for example, Japanese Patent Application No. Hei. 6-311695 is known.
FIG. 9 is a schematic cross-sectional view illustrating a conventional spindle motor in which a hydrodynamic oil radial bearing is adopted. In the drawing, reference numeral 1 denotes a bracket having a flange fixed to a base of an information apparatus body such as a hard disk; numeral 2 denotes a stator fixed to the bracket 1; and numeral 3 denotes a shaft fixed to a central portion of the bracket 1, the shaft 3 having a shape in which a cylindrical portion and a disk-shaped collar portion are combined. The cylindrical portion of this shaft 3 has grooves formed therein for generating dynamic pressure, and serves as a hydrodynamic oil radial bearing 3a, while the disk-shaped portion of the shaft 3 has dynamic-pressure generating grooves formed in its upper and lower surfaces, and serves as a hydrodynamic oil thrust bearing 3b. Numeral 4 denotes a housing for retaining a disk (not shown), which is a recording medium, and numeral 5 denotes a rotor which is secured to an inner peripheral surface of the housing 4 and is formed of an annular magnet. Numeral 6 denotes a sleeve member secured to an inner peripheral surface of the housing 4, and this sleeve member 6 has an appropriate gap with each of the hydrodynamic oil radial bearing 3a and the hydrodynamic oil thrust bearing 3b, and receives a radial force and a thrust force due to the dynamic pressure generated between the sleeve member 6 and the shaft 1 when the motor rotates. Numeral 7 denotes a thrust holder fixed to the sleeve member 6.
The conventional spindle motor is constructed as described above, and the rotor 5 is rotatively driven by electromagnetic action between the stator 2 and the rotor 5, which in turn causes the housing 4, the sleeve member 6, and the thrust holder 7 to be rotatively driven with respect to the shaft 3. In conjunction with the rotation, dynamic pressure is generated at the hydrodynamic oil radial bearing 3a and the hydrodynamic oil thrust bearing 3b, so that the sleeve member 6 and the thrust holder 7 rotates at high speed and with high accuracy without coming into contact with the shaft 3.
In the above-described spindle motor having the hydrodynamic oil radial bearing and the hydrodynamic oil thrust bearing, there have been problems in that since oil is used as a lubricant, if this spindle motor is applied to a large-capacity storage device for which high speed and high accuracy are required, the disk may possibly become contaminated due to the scattering of oil, and the rotational accuracy may change since a change in the properties of oil can occur due to a temperature change caused by the heat generated by the motor.
These problems occur since oil is used as the lubricant, and as a means for overcoming these problems, it is generally conceivable to use a hydrodynamic gas radial bearing and a hydrodynamic gas thrust bearing. The hydrodynamic gas radial bearing and the hydrodynamic gas thrust bearing are those in which oil is replaced by gas as the lubricant in the hydrodynamic oil radial bearing and the hydrodynamic oil thrust bearing. It is true that if the hydrodynamic gas radial bearing and the hydrodynamic gas thrust bearing are used, the problem of contamination with oil can be overcome; however, since the load carrying capacity is small, it is necessary to increase the length of the bearing, which gives rise to another problem that the outside diameter and dimensions become larger than those of the hydrodynamic oil bearing.
Further, with the hydrodynamic gas radial bearing, it is also necessary to increase the rigidity and processing accuracy of the bearing retaining portion, so that there has been a problem in that it is difficult to realize a compact and inexpensive spindle motor.